Passive liquid crystal display devices are made of a layer of liquid crystal disposed between opposed electrodes activatable in segments. Light passing into the device through the front electrode, which is transparent, is internally reflected from the back of the device, e.g. from a reflective film or coating. As is well known, the light entering at activated segments of the device is modified by the liquid crystals to provide a contrasting visual effect relative to other areas. To obtain a practical display that corresponds to the shape of the activated regions, a contrast ratio of about 2:1 is required where the display and background are of the same color. The contrast ratio varies proportionally with ambient light, assuming a constant electrode potential. For this reason, where liquid crystal devices are desired to be used at night or at low ambient light levels, efforts have been made to provide additional lighting.
Those suggestions which employ an incandescent light source, or other point light source, must contend with the problems of nonuniformity of illumination. Examples of Castleberry U.S. Pat. No. 4,212,048, which employs a transmissive diffuser; Brooks U.S. Pat. No. 4,206,501 and Eberhardt U.S. Pat. No. 4,229,783, which employ diffusers for bulbs located behind the display, Eberhardt having an additional reflective front surface to facilitate diffusion of the ambient light; and Borden, Jr., U.S. Pat. No. 3,748,018, which employs a reflector to direct light from the source onto the rear surface of the display.
Others have sought to employ electroluminescent lamps behind the display devices, with selective reflectivity provided in the ambient light mode by means of a dichroic wavelength selective reflector (Aldrich U.S. Pat. No. 3,869,195), or a microlouvered light control film (Myer U.S. Pat. No. 3,811,751) or by utilizing the reflective rear electrode of the EL lamp itself (Saurer et al. U.S. Pat. No. 4,138,195). These have various drawbacks such as substantial expense and lack of sufficient contrast in one or the other of the modes of operation.
It is an objective of this invention to provide a simple, low cost and effective device for uniformly illuminating, from the back, a passive liquid crystal display for viewing at night or under conditions of low ambient light by illumination from the rear; it is also an objective to provide a device useful with a passive liquid crystal display, to allow the display to be seen in all levels of light and darkness; a further objective is to provide an improved transflector and method for its fabrication.